Add more prefix support in <dir> element

Added two prefix modes:
  "relative" that makes the relative path be relative to current file
  "cwd" for relative to current working directory which implies current behavior.

Resolves: fontconfig/fontconfig#15
2 jobs for <span class="ref-name">issue/15</span> in 6 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #26473
shared-build

00:06:47

passed #26474
static-build

00:04:49